A ring resonator gyro typically has a laser which sends a continuous very high intensity beam of light in two opposite directions around an optical resonator ring, means for frequency shifting the laser light so that the light circulating in the ring is in resonance in both directions and means for deriving any angular rotation of the ring by monitoring the frequency difference between the light travelling in opposite directions.
As a result of the very high intensity light beam, non-linear effects in the resonator become significant in the operation of the gyro. One such non-linear effect is the Kerr effect which is caused by the refractive index of a material being dependent upon the intensity of light travelling through it. Thus, if there is a difference in intensity of light travelling in the two opposite directions around the ring resonator, then there will be a difference in optical path length due to the difference in the refractive index in the two directions. The refractive index differences may be represented as follows: ##EQU1## where I.sub.cw and I.sub.ccw are the intensities of light in the clockwise and counter-clockwise beams respectively and .DELTA.n.sub.cw and .DELTA.n.sub.ccw are their corresponding refractive index differences. The optical path length difference causes a gyro bias which varies as a function of the relative intensity. To overcome the Kerr effect induced gyro bias, the intensities of the beams in the opposite directions have to be very accurately matched.
An acceptable upper limit for a gyro bias is 10.degree./Hr which would be caused by a 1 microwatt offset in the input power for the opposite directions. Since the power in the ring resonator gyro is generally 10-100 milliwatts, then the intensities would need to be matched to one part in 10.sup.4 -10.sup.5 to reduce the bias to match this limit. Equalisation of the intensities of the two beams to this degree of accuracy is very difficult to achieve in practice. For example, if samples of the beams travelling in the two directions are taken and compared directly, differences in the beam sampling arrangements and differences in the sensitivity of detectors used to monitor the two beams are likely to be much greater than the degree of accuracy required.